Need help with sculpting a shield
For a base mesh I would use Sphere (12 segments), upper part scaled on Z in Edit mode. If you don't like curvature, scale Z to 0, select centre vertex and with enabled Proportional Editing O (and chosen desired curvature) move up on Z axis.




For spiral enable addon Extra Curve Objects. Add new object Spiral and tweek parametrs in properties. Add Shrinkwrapp modifier > Target your "base" mesh.



Add Plane, Scale it down into thickness os spiral shape, add Array > Fit Curve, Curve > Spiral, Shrinkwrap > Sphere, Solidify, alternatively Bevel modifiers (see screen).



Thorns - I choose hand modelling, just apply Array and Curve modifiers, disable Shrinkwrap and extrude a few vertices for thorns. In screen I used move vertices, dont do that :)



Tip: just by selecting inner side and Scale you can get wide of spiral thinner at centre and thicker on outer side of shield.



For 3D print you will need manifold object. Blender has an add-on 3D Print Tool that can help to show problems. To get it right seems to me better extend spiral ornament and cover whole shield with shrinkwrap - like retopology, so you get nice wire for print.
When I tried to Merge by Boolean or Remesh it comes with a lot of issues.



Another way

For 3D print would be ideal to start with Curve profile of shield. Add Screw modifier to create shield and use Simple Deform > Twist modifier to create spiral topo. Like that you will get shield shape with Spiral wire so simple extrude of one loop can does the job, but I wasnt able to create spiral in shape from your image. But you can try your luck with that.






Mesh is quite dense so I just pushed vertices up here. You can also create Shape Key before you start, like that you can create more designs on shield or you can any time to get back into clean surface.